A female monarch lays her eggs one at a time, each on the bottom of a different milkweed leaf. She can lay several hundred eggs in her lifetime.

The photograph at left is highly magnified, showing both a monarch egg and the fuzzy hairs of a leaf.

Inside the egg, the caterpillar begins developing. In 4-5 days, the egg hatches, the young caterpillar climbs out, and it begins eating the milkweed leaves. Bitter chemicals from the milkweed accumulate inside the caterpillar. Birds and other potential predators have learned to avoid eating monarch caterpillars and adult monarchs because this substance makes them sick.

As the caterpillar grows, it molts (sheds its skin). Each new molted caterpillar is called an instar. A caterpillar will molt five times before entering the next stage of its life. At right are two different instars.

Another insect found on the milkweed plants are milkweed seed bugs. Shown here, some of these bugs share a plant with three monarch caterpillars.

After about two weeks of eating, a caterpillar will crawl to a spot to pupate. The transformation from the caterpillar stage to the chrysalis (pupa) is shown below.

The chrysalis skin hardens and the butterfly continues its metamorphosis inside. In this stage, it cannot eat. If you look closely at the photograph immediately left, you can see the developing wings inside the chrysalis.

As shown below, about 10 days after the pupation stage began, the chrysalis skin becomes transparent and the adult butterfly emerges.

Adults do not waste any time finding a partner with which to mate. In the photograph on the left, the female at the top was released less than 1 hour prior to mating. She can begin laying eggs in about 5 days.

In southern Florida, there can be up to six generations each year. They do not migrate.

In most other areas there are five generations each year. Most adults live only a few weeks, but the generation that migrates south can live over 6 months. As winter approaches in northern Florida and in other parts of the United States east of the Rockies, monarchs migrate to Mexico. Some of these monarchs fly as much as 3000 miles to roost in the same trees as their ancestors. Monarchs west of the Rockies migrate to trees along the coast of California. When spring arrives, the overwintering monarchs in both California and Mexico mate, lay eggs, and then their offspring begin the migration back North.